In many signal processing applications there is often a need to estimate an amount of delay introduced by circuitry. Taking modern wireless transmitters as one of many possible examples (e.g., wireless transmitters in mobile phones, or in computers with built-in transceivers), knowledge of the transmitter's delay (i.e., between input and output) may be an integral part of techniques applied to make any of a number of types of measurements in a closed-loop configuration. Different types of measurements can be made with such a configuration, such as (without limitation): transmitter power measurement, transmitter gain measurement, transmitter phase measurement, transmitter IQ mismatches (i.e., mismatches between amplitude and/or phase of an In-phase and Quadrature signal pair), and transmitter adaptive pre-distortion.
In a closed-loop configuration, the feed-back signal typically is a delayed but accurate (or at least highly similar) copy of the base-band transmitted signal. This delay will vary with process, transmitter settings, temperature, and the like. The loop-back delay can severely degrade the closed-loop measurement if not compensated for.